The present invention relates generally to hydrophone apparatus for detecting pressure variations in the surrounding medium, and more specifically to improved high sensitivity hydrophone apparatus of the streamer type wherein sensing of pressure variations occurs along the length of an elongated member.
It is well known in the seismic detection and underwater sound sensing arts to employ a plurality of hydrophone elements positioned along an extended marine cable or streamer for achieving increased sensitivity and acoustic signal detection capability. Such streamers may be hundreds or thousands of feet long, and are typically operated in tow behind a marine vessel. Detection of signals throughout a volume of water is accomplished by towing a streamer along a predetermined path. Obvious advantages in the speed at which a marine survey can be made and in minimizing the cost of conducting such a survey accrue from increasing the towing speed.
A significant limiting factor on the effectiveness of a hydrophone streamer in detecting weak or distant signals is noise which may at least partially obscure or mask the signals. There are various contributing sources of noise, some of which are internal to the transducers and associated electrical and electronic components. Other significant sources of noise include: (1) vibrations of components within the streamer resulting from mechanically transmitted movements of the streamer and the restraining and/or towing lines, (2) direct transmission of pressure waves in fluid fill in the streamer induced by turbulent flow of water relative to the streamer, and (3) scattering of fluid fill velocities by interior streamer components. The level of noise originating from the last noted sources is generally directly related to the physical configuration of the streamer apparatus and the towing speed.
It is also advantageous to minimize the diameter of the streamer apparatus to reduce the handling problems of long streamers. However, the degree to which the size of a hydrophone streamer can be reduced is limited because reduction in size results in reduction in sensitivity. As size is reduced, a point is eventually reached where internal transducer and electronic noise exceed mechanically induced noise. Thus, there are practical limits on reductions in hydrophone streamer size.
A variety of means and techniques have been employed to reduce mechanically induced noise sensed by transducer elements in a hydrophone streamer. Noise resulting from turbulent flow relative to the streamer structure can be minimized by employing a smooth, streamlined outer configuration, and designing the streamer so as to minimize local and overall deviations of the longitudinal axis of the streamer from its direction of travel. Typical prior art techniques for minimizing deviations of streamer axis from direction of travel comprise achieving substantially neutral buoyancy by means of float members as shown in U.S. Pat. No. 3,696,329 issued to G. D. Hazelhurst on Oct. 3, 1972, or lighter than water fill materials as shown in U.S. Pat. Nos. 3,480,907 and 3,518,677 issued respectively to J. D. King on Nov. 25, 1969 and E. F. Florian on June 30, 1970, or combinations of air and heavier than water materials within the streamer as shown in U.S. Pat. No. 3,868,623 issued to F. R. Abbott on Feb. 25, 1975, or hydrodynamic depth control units as shown in U.S. Pat. No. 3,660,809 issued to R. H. Pearson on May 2, 1972.
A degree of isolation of the transducer elements from mechanically induced noise can be achieved by means of suitable filler materials surrounding the elements as shown in U.S. Pat. Nos. 3,480,907 and 3,518,677. In addition, a degree of isolation may be provided by hydrophone saddles, mounting tubes or cradles as shown in U.S. Pat. Nos. 3,518,677, 3,660,809 and 3,696,329 respectively. However, as performance requirements have become more demanding, it has become necessary to further reduce the noise levels due to the direct transmission of turbulent flow pressure waves and scattering of induced fluid fill velocities in hydrophone streamers, and to achieve more effective mechanical isolation of the transducer elements therein. In response to this requirement, the applicant has discovered and developed uniquely configured hydrophone streamer apparatus wherein substantially concentric tubular members are employed to achieve exceptionally sensitive and low noise performance.